Silicon carbide refractory article



Sept. 2, 1952 J. P. swENTzEI.

SILICON CARBIDE REFRACTORY ARTICLE INVENTOR.

L E z T N E W S P N H O J ATTORNEY Patented Sept. 2, 11952 SILICON CAR-RIDE REFRCTORY RT'CB Joln P. Swentzel, Niagara'Eallsgi'N..fY. assigner" to The Carhorundum Company, NiagaraiFalls', N^,-Y.,.a.corporation of Delaware Application December 8, lgfllrserialNo: B12583511 'Ih-isinventionrelatesto bonded siii'corrcarbiderefractory compositions.

-More particularly itrelates. to bonded. silicon carhid'e refractory shapes of a. composite'nature made from an' integral. combination ofbo'nded silicon carbide. compositions which shapes not only possess highA thermal conductivity and s'trength'at elevated' temperatures, and are re- 'sistantf'to oxidation. butfalso are especially re.- sistanttoheat shock. and at the same time can be used' at high temperatures without. imparting discoloration or otherwise'stainingf objects with which the refractory comesinto contact;

In'-my'copending application Serial'No.. 11,356, led February 26',` 1'94'8, Ilhave disclosed and described the making of honded silicon carbide refractory articles which possess high thermal conductivity. and' strength at elevatedtemperatures; are especially resistant to. oxidation' under severe oxidizing Vconditionsandr are satisfactory foruse at. high ternperatures.l Withoutr imparting discoloration to objects or materialswith. which the refractories come in contact. during. use with high temperatures.r Although 'the refractory bodies thereindisclosed show a marked' improvement over' the.- more conventional clay or. glass bonded1 silicon carbide refractory hodi'es-intheir resistanceto heat sho,ck they are still limited to some extent in their application because. of their stilli limited resistance toheat shock.. In other" Words,v the. refractory hodies, thereinv described'. still' tend to crack. or spall when exposed tosudden changes in temperature, thereby resulting in failurel of. theY refractori'esvwhen'used in'fconditions such as those encountered" in or'f'urnaces where the uctuations in temperature lare frequent orexces'sive in. amount. The 'silicon carbiderefractories therein disclosed can bebriey described as being composedy substantially of` granular silicon carbide bonded by a silicon nitride hondV which isobtained by 'ring an article Yof the desired shape composed Vof granular silicon'carbide and iinel'y' divided' silicon metal'in an. atmosphere of. nitrogen or, some other nonloxidizing, atmosphere. containing.' n-

trogenl at atemperature sufdcient. to convertthe silicon. 'metall to 'interstitiall'siliconnitride having "the chemical formula SN4..Whch bonds the. sili' confcarbide:v granules '.to. form a. refractory shape of the desired. strength. andrefractoriness..

`Lhave alsof been ableV to make-silicon. carbide refractory articles of satisfactorily.. high: strength andiresistance to oxidation'. ati high: temperatures and# which;Y in addition; are.- substantiallyf more resistant tovspallingzon otherwise breaking;` under 7"y Claims. (01... 1549-43);

heat; shnclr; vbyfrorming; the: articles on a. of granular silicon carbide and a ferrosilicon or'ferromanga'nese siliconvrneta-lr` incnely divided form; tlei .articles beingf flred annui-oxidizing?. nitrogenous: atmosphere: whereby a nitride-.bond is formed' for the; silicon. carbide; particlesa.. suclr. articlesl possess f a: high` resistance'. tir-meals.-V

age* under." heat. shock. in'` addition.,` torathigh strength; and.' resista-nce-V to:v oxidation-f them-.hare been'. found". to.- have the disadvantageIdueztovthe presence` ofi the'. added. iron` and: manganese'. conf'.- stitu'entsfof the bond. off formingf afg-laze om the surface. ofV the article:` which. often: .-impartsi discoloration vandi stainingzto.- any' objectsfcoming in contactwith. the refractory' atv highftempera.- tures.. For-f that .reason the refractoriies: in.- spite oftheirfresi'stance toiheatshockmaveheenound dnacceptab'le.L for? manyy uses",1 for: example; as seiiter tiie.- irr'kilnslv Where: White` ware.' on other colored# articles. are: beinghred'.. 'Alsorthewconfventi'onal. setter.' tile refractory."- waslnlcoatings. suchxV asialuminafrefractory slurries doanot satisfactorily adhere; to refractory tilevorsaother shapes which. are" formed?. fromz-.amixtureosiliicorr carbideandra nitride: bond; developed:A from fer-rornan'ganese1..silicomory comparable ferrofaliloys', thereby making the tileunsatisfacixiry-for use! under'. conditions'. Where; it. is. .desired :to: objects without surrounding.z redding', materialfn It1 isf an. object.. off the present-invention topwvide an improved silicon carbide. refractory. anticle'4 which isv highlyy resistantltos heat'shockor rapid:` fluctuations in'. temperatura It iisT afurther'object. oft thefpresentimentlun to: provide.I an. improved'.l bonded.V siliconi carbide article".` which will:y stand upv underheayy' jloads atxhigh temperatures..

It is a still further object to provider@ bonded silicon.. carbide` article in: which: theibondi 1st-smell as the silicon carbide component isioifa refaetorinessl.considerablyabove thatfof suchfcommon bonding. materials asavglas'sg'. porcelain or thelike.

.It is a rurther object. to' providear bonded-siii@ conl carbide article'zwhichz'inadditiorrtmtheabove properties is non-stainingJi'ni ch'aracter'aftihi'gh temperatures.

Itlis'- avv-still further objcttolprovidewasilicon carbide body f that#avoidsl thefdisadfantagesof. silicon' carbide refraci'fori'esffof thefprorarii.

A @therv objects-and'advantages accruing* ii'rri the present invention'-v will* become" obvious"andl apparent as the descriptionproceeds:

In accordance jwithi the present invention, bonded silicon carbide articles ofV a composite but. integralnature are inadeiin 'whichthemain body portion of the article is composed of granular silicon carbide and a silicon nitride bond containing certain glaze-forming added ingredients such as compounds of iron, manganese and the like. The main body of the article is integrally bonded to a surface portion composed of granular silicon carbide bonded by a silicon nitride bond, the surface portion of the article being substantially free of the glaze-forming constituents present in the interior of the article. The nitride bond in the main body portion of the article is obtained by using a metal alloy such as ferrosilicon or ferromanganese silicon in the raw batch from whichrthe main body portion is made and the nitride bond in the surface portion is obtained by using silicon in the raw batch from which the surface portion of the article is made. The silicon is converted to silicon nitride in the process of iiring the article, and at least the silicon,` if not all the alloy, is also converted to a nitride bond at the same time. Y

Since the present invention is especially adaptedto the making of bonded silicon carbide setter tile it is to be understood that in referring to a bodyportion and a surface portion that the surface portion need not extend over and include the entire exterior of the article. For example, a composite setter tile can be made in which one of the expansive iiat surfaces of the tile which normally would come. in contact with objects being put -through a kiln isprovided with a surface layer or portion of the above description to provide the article with a non-staining upper surface whereas the underV or opposite surface of the tile can be composed of the main body composition. In other Words; a composite setter tile canbe made with one or both of itsexpansive at sides faced with a surface portion composed of silicon carbide particles bonded by a silicon nitride bond obtained by conversion of silicon metal to silicon nitride, and with the main body portion composed of silicon carbide bonded by the nitrided product of -a silicon` alloy such as ferromanganese silicon or ferrosilicon. Similarly, a tube orother shape may -have either its inner and/orits outer curved surface., faced with a bonded silicon carbide body Ahaving a silicon nitride bond with the 'balance .ofthe body structure formed of silicon carbide bonded by the nitride product of a siliconalloy ofthe aforesaid type. f

. Y Inorder that the invention may be more clearly understood `reference is madeY to the figures of .the .drawing which present one illustrative embodiment of the present invention, and in which,

Figure .l is a perspective View of a silicon carbide setter tile made in accordance with the present invention, t Figure 2 is a sectional view through the, line 2-2 of Figure 1; and

Figure 3 is a View similar to that of Figure 2 showing a modiiied form of setter tile made in accordance with the present invention in which one surface only of the tile is provided with a nonstaining surface portion.

In order that the invention may be more fully understood the following specic examplesl are submitted for illustrative purposes and indicate the manner in which composite silicon carbide articles bonded by a nitride bond can be formed in accordance with the practices and principles of the present invention.

EXAMPLE r Silicon carbide setter tile 14 x 15" and approximately l thick, such as that illustrated in Figures 1 and 2 of the drawing, have been made by the joint use of the following compositions:

Composition A Parts by weight 14-36 mesh size silicon carbide grain 48 mesh and iiner silicon carbide grain 36 260 mesh size and finer silicon powder 16 Dry lignone 5 Bentonite gel 6 Composition B Parts by weight The ferromanganese silicon alloy used above is one containing approximately 48% silicon, 20% manganese, and the remainder mostly iron. Ferromang'anese silicon, or ferrosilicon, alloys of other compositions can' be used with satisfactory results, or a mixture of powdered silicon metal together with powdered iron or powdered iron and manganese can be used, It is desirable however that the silicon content amount to at least 50% by weight of the total metalbondin'g ingredients, and usually 75 to 98% of the total metal content of the raw batch Von a dry weight basis, the other alloying metals amounting to 2 to 25% of the metal.

Composition A above is used in the ,manner described below for making the surface portions l and 5 of a setter tile such as that shown in Figures l and 2 and Composition B is used as described below for making the body or interior portion G of the tile. e

The ingredients of each of the Compositions A and B, with the exception of the bentonite gel in each case, are dry mixed in a tumbling barrel for 15 minutes to insure good blending, after which they are wet mixed for an additional 20 minutes in a kneader mixer. Each of the mixes is wet to the proper pressing consistency with the bentonite gel which is made of four parts of water and one part dry bentonite powder. The bentonite gel in'Composition A serves to take up the finely divided silicon powder of the mix and in Composition B it serves to take up the iinely divided ferromanganese silicon powder, which metal powders are otherwise very fluily and extremely diicult to handle, and distributes those powders evenly and uniformly throughout the molding mixtures to provide well blended batches of suitable consistency for molding.

A setter tile such as that shown in Figures l and 2 is molded by iirst placing inthe mold a sufficient 'amount of Composition A to provide when iinally pressed a layer of that material approximately thick in the bottom of the mold. The material is leveled oi and uniformly distributed over the bottom plate of the mold and, if desired, subjected to a light pressure sufficient to rm the material in position so as to avoid displacement during the subsequent addition of the next layers of material to the mold. A suiicient amount of Composition B is then placed in the mold over the lightly pressedv material of Composition A to provide when nally pressed a layer of material of Composition B approximately Ik" mr-.tmclmessi 'frheimateriarnethen distribeinen.

exnlyoizer'the area mthe'mold and;;;again;if 1de. sirech; 'li-g'h-tlyi lpressed; into., position, ia'fterimhch anothertlayerof- `of-Comuosition A sulcient. toiprovidefanother layer approxilnatelyritt"V thick whennallypressed; placed ori-top of the lightlw pressed Composition: A.. After the lastlayer Aof materiah hasbeen leveledfoff and uniform-ly. tributed over lthe mold areafthe:upper-fmoldgplate isputin positionontopfof. the mold lcontents 'and they mold placed infa, hydraulicgpressand .the tile formedbyrpressingatsa.,pressure inoexcessiiof* 5.0.00.:A poundsperzsquareinch. During;the,final.pressing oithe article the three .layers of material-.constituting the entiremoldxcontents. 'arezforcedtogether to form an integral bodyxin Which'ComfnositionB formsgamiddle coreorbodyportion. appIoximately--Ll/a." thick'Lfaced-andhonded 'onboth sides to'lsurfacelayers of Composition A, each approximatelys* thick.. .moldedfshapegis removed, from the mold, .v driedj and placed. vinea,

muillef furnace;` The-:normali atmosphere inA the muillcfisreplaced. baia continuousfstream of. commercial grade nitrogenhavingY apurity of 99;.7 '921, after which` lthe temperature: of the furnace chamber isgradually; raisedy overa. period oil'severaljhours to1400 C.. and held at thatateinpera.- ture-,f for 12- hours, while a: continuous'stream. of. nitrogen is. fedyintothe furnace muffle.. The :furt nacewlth the; nitrogen. still ilowing-is allowed; to: cool-to room. tempera-ture.. or to. a .temperature convenient .for handling the. tile whereupon.. it; is removed from theffurnace: ready foruse.r Such tilearev highly suitable-tof use in kilns for. the ri-ng of White Ware andthe like Where; thecycle. of-V` heating is; relatively rapid and havethe dual advantageousv features Iof notv only not .stai-ning the Ware coming in contact with the tilebutalso. of;` standing up under the rapid fluctuations-'in temperatureinvolvedu in the operation of: such typekilns..

EXAMPLETII.

6*. siomof atleast thasilicon, .fsome;:if:.notiia1li ofazthe;.al.lovingmetalstheirawzbatoh 'to mtrides l article'.

'The modulus of rupture fot siliconzcarbide' articles :made in ,1 accordance. lwith-fthe :composiff tions?andproceduressetforth inlExamp'le Allalrove v main :bodyf portion ofi refractory'articleszmadeemg accordance with thegpresent invention-canapa varied` with, fsatisfactory resultsv 'between 43m-idr 40% byf'dry weightxof .theravvzbatchimix amount of silicon used inthe rawzbatchiifmm whichthe surface portions of thearticle arefmade can also Vary betWeenfl and.,40%.drM-Weightsfofz' theraw batch.. However, infeaclrportionzofthe article itis usually desirable.v to use .the neighf bor-hood of 8-40% combined weigV-htnof; :metal nitrideforming *bondingl vingredients in. lorder tn provide the article'wi-th asatisfactory vtherrnat conductivity,V highlhot strength and.resista-noeste.,y

"Referringagain tolFigure,3,.ia.tile of. that. type;

made-in a .manner similar` tov that= described above ..for the making of the tile illustrated in Figures 1 and 2 except-that the. initial charge o fj Composition A is not. placed in the mold and the. amount of Composition B' material'` placedin the mold is, correspondingly increased to compensate for. the. absence of the. lower layer of Composition'B so that the finally pressed. tile-Will have a thickness in the neighborhoodof l", a1- though itwill consist of. only 2 diierent strata of'material. Obviously the thicknessofthe vari ous layers. of materiall making upY thev composite. article can 'be varied. considerably from the amounts. specied in the. example, above without departing from the sphere of the present invention andV will be. dependent upon the overall thickness ofthe tile desired as well asiupon the. relativeV importance of theinon-staining feature. and the resistance to heat shock in` thenal article'all'of' which will depend upon the ultimate use for which the article isintended.

"It is of interest-to `note that there isno change in the volume of the articles as a'result of' the ringoperation. It is found upon weighing the moldedand driedf articles 'beforeandfafterY the. ring operation that they undergo a gamin Weightof approximately 68%, which, if` calculated on' thevbasis of theV combined silicon ami silicon alloy-content of thearticle; and corrected for loss of volatile 'matten 'indicates' a` "gain Vin weight offthef'fbond ofsuil'i'cient magnitude to that: thererfhasbeen a. substantial lconverheatshock..

Thealloying metals-:other than the; silicone@ Y the; ferromanganese siliconandi?errosi-liconallons;

havebeen ref-erred'to herein as.Lglazefforrning gredients. This term has beenapplie'd tot-'these alloying metals because -ofr .the observation that;v when silicon. carbide bodies'.composed-entirely :of compositions gsuch as Composition B- I of; Example I arepfusedin which the fnamedalloyszare .included as a source of the nitride bond such articlesde. velop at the surface duringuseacertain- .amount of-aglassy material vorglaze Whichttends to. stain orotherW-ise discolor objects with which the-bodyg comes in contact,.whereas similar siliconcarbide bodies such. as. Composition A of. Examplev lLin which the nitride bond isobtainedfromfpowdered silicon in the rawbatch--do-not developtheesaine. glazeunder the same-firing conditionsiorisimilar conditions of use. It is believed that in articlesoi the present invention. the presenceeof theseadditional metal ingrediehtsiin the mainbody portion of the` article, either of themselves or. imcon.- junction with small amounts vof silica. or. .other impurities presentin the. silicon. carbide.; granuff lar material, tends to greatlyy increasethef rfa-.vr sistance .Y of the. resultant article to; cracking, yor breakage when exposed to .heatshock At-the same time the connement of such sofcalledz glazefformi'ng.'constituents-to: the interior ofthe.- article serves thepurpose of. keeping ithersurfaces. oi. the. article freev or. substantiallyfreeiof anyfuny desirable glassy. substance. which might.-stain KA.or discolor objectsplaced incontact with-theslcotlyf` during use..

I vhavefound. thatl highlxr satisfactory.r resultsare` to. be. obtained. by .using commercial-grade. silicon metal ground.tosuitableneness.. .Ana-l.- ysis of a commercial. grade of.. siliconwhichl period of time when a commercial grade silicon powder ofthe above typeY is used the silicon should be fine enough to pass through a screen of around 200 mesh size (U. S. Standard sieve) and nner, which is around '70 to 90 microns in particle size. Still morerapid nitriding isobtained when the silicon is of a neness in the.

neighborhood of 10-20 microns and finer. Satisfactory conversion of the silicon to silicon nitride bond also has been obtained with pure silicon (99.8% silicon), although it has been found that When the pure form of silicon is used, the period of time required for nitriding is much longer than that required for nitriding articles of similar size and shape formed of commercial grade silicon of the same degree of neness provided the other conditions of the nitriding operation are the same. The rate of conversion of the silicon to silicon nitride when pure silicon is used can be increased by a reduction of the silicon to a ner particle size. It also has been found that the rate of conversion of the pure form of silicon to silicon nitride can be increased by adding a small percentage of iron powder, say, in the neighborhood of to 1% by weight, which is the amount commonly found in commercial grade silicon, to the pure silicon. The greater ease of conversion of the silicon to silicon nitride when commercial grade silicon containing the aforementioned impurities' is used as compared with the nitriding action obtained with pure silicon is to be attributed, therefore, to the presence of the small amount ofA iron impurity commonly found in the commercial grade of silicon metal. Such an amount is insumcient to provide any appreciable the use of other non-oxidizing atmospheres containing nitrogen. For example, commercial annealing hydrogen, Vwhich has an approximate analysis of 93% nitrogen and '7% hydrogen, or ammonia gas can be similarly used in place of nitrogen. l

Although I have indicated a number of nonoxidizing, nitrogenous gases which can be used as a direct source of nitrogen during the nitriding operation by carrying out that reaction in a reaction chamber into which the nitrogenous gas is continuously passed, itis to be recognized that lif desired the nitriding reaction can be effectively carried out in other manners providing that the atmosphere immediately adjacent and within the article being nitrided is maintained substantially non-oxidizing in character and providing an ample supply of nitrogen is furnished the article. For example, in ring bodies or molded-shapes in accordance with the present invention in which a nitride bond is to be formed the body can be embedded in a suitable mixture of coke and sand and the article red at the aforementioned temperatures. The silicon of the surface portion, and at least the silicon component of the alloy metal in the main body portion, are converted to silicon nitride by the nitrogen of the air, which'penetrates the embedding mixture and reacts with the silicon contained in the articles embedded therein. The coke of the embedding `mixture combines with the oxygen of the air and forms carbon monoxide before the free oxygen has an opportunity to reach the articles being fired, so that the gases penetrating to the article are substantially a mixture of nitrogen and .carbon monoxidel a Under these `conditionssubstantiallyall theisilicon will V combinev with thenitrogen ytoform silicon nitride; Obviously,- suiiici'ent coke must be provided-inthe mixture to providean excess of carbon, so that carbon monoxide will beformed rather thancarbon dioxide,v and so that no free. oxygen willzbe present. The embedding material must be of sufficient quantity to function inthe prescribed manner. As further assurance that an ample supply of oxygenfree nitrogen is freely available to'each articlel being thus fired, each individual shapemust be spaced from adjoining articles with ample intervening embedding material. Otherwise, satisfactory'nitriding does not takeplace in a reasonable-length of time. 1 i

Moreover, although a specific temperature has been recited inthe aforementioned examples for the nitriding operation, and best results, i`. .-e., most eiiicient and thorough conversionof the silicon to silicon nitride are found to take place when the nitridingl operation Vis-performed at temperature ranges slightly below .the melting -pointof silicon (1420'.1C.), as, for example, 1350-1400" C'., satisfactory nitriding has been obtained at temperaturesas low as in the neighborhood of '1300o C. Furthermore, during the nitriding operation,v and' particularly after the nitriding has pro-1 grossed forsoine time, the temperatures can be raised well above the melting point of silicon as further assurance of the substantially complete conversion of the silicon to silicon nitride."

The articles made in accordance with the present invention may be molded by any ofthe wellknown methods including pressure molding, machine tamping, hand tamping, jolting, vibrational tamping, air hammer flator edge-tamping, or slip casting. Y

It is to be understood :that the products of the present invention in its various modications are not limited to any specific field or elds of use such as might be defined by the specific examples previously set forth. The products can be made in any desired shape. They are, therefore, not only suited for many of the uses for which industrial refractories are required, including bricks, blocks, setter tile, muiies, kiln furniture, and special shapes for application in and around furnaces andl other high temperature equipment, but they are also well suited for `many specialty high temperature applications, such asjet engine combustion chambers, linings forexhaust nozales, rocket combustion chambers and exhaust nozzles, turbine blades, stator blades, lens fusionI blocks, spark plug bodies, and the like. They are also suitable for the fabrication of laboratory Ware, including combustion boats, crucibles, burner holders, and other shapes. Obviously, that composition constituting the surface portion of the article may extend over the entire exterior of the article or only that selected part of the exterior which it is desired in use should be substantially free of glaze or glaze-developing material.

Having thus described the invention it is desired to claim:

Vl. A bonded silicon carbide refractory article comprising a surface layer of silicon nitridebonded silicon carbide, the silicon nitride thereof having the chemical formula SiaNi, said surface layer being integral with a main body portion comprising silicon carbide bonded ley-silicon nitride of the chemical formula Si3N4 and con-L taining 2 to 25% of iron and manganese.

2. AA bonded silicon carbid@refractoryarticle 9 comprising a surface portion comprising silicon carbide grains and 4 to 40% of a silicon nitride bond in Which the silicon nitride has the chemical formula Si3N4 and amain body portion comprising silicon carbide grains bonded by the nitride reaction product of 4 to 40% of ferromanganese silicon.

3. A bonded silicon carbide refractory article comprising a surface portion comprising silicon carbide grains and 4 to 40% of a silicon nitride bond in which the silicon nitride has the chemical formula Si3N4 and a main body portion comprising silicon carbide grains bonded by the nitride reaction product of 4 to 40% of ferrosilicon. Y

4. A bonded silicon carbide refractory article comprising a surface portion comprising silicon carbide grains and 4 to 40% of a silicon nitride bond in which the silicon nitride has the chemical formula SiaN4 and a main body portion comprising silicon carbide grains bonded by 4 to 40% of a silicon nitride bond containing 2 to 25% of iron and manganese, the silicon nitride having the chemical formula SisNii.

5. A bonded silicon carbide refractory article comprising a surface layer of silicon nitridebonded silicon carbide, the silicon nitride thereof having the chemical formula Si3N4, said surface layer being integral with a main body portion comprising silicon carbide bonded by silicon nitride of the chemical formula SiaN4 and containing 2 to 25% of an alloying metal selected v i from the group consisting of iron and a combination of iron and manganese.

6. A bonded silicon carbide refractory articlel prising silicon carbide grains bonded by the nitride reaction product of 4 to 40 %V of a silicon alloy selected from the group consisting of ferromanganese silicon and ferrosilicon.

JOHN P. SWENTZEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 866,444 Egly Sept. 17, 1907 1,345,377 Linbarger July 6, 1920 2,118,789 Fisher May 24, 1938 2,364,108 Swentzel Dec. 5, 1944 2,406,275 Wejnarth Aug. 20, 1946 

1. A BONDED SILICON CARBIDE REFRACTORY ARTICLE COMPRISING A SURFACE LAYER OF SILICON NITRIDEBONDED SILICON CARBIDE, THE SILICON NITRIDE THEREOF HAVING THE CHEMICAL FORMULA SI3N4, SAID SURFACE LAYER BEING INTEGRAL WITH A MAIN BODY PORTION COMPRISING SILICON CARBIDE BONDED BY SILICON NITRIDE OF THE CHEMICAL FORMULA SI3N4 AND CONTAINING 2 TO 25% OF IRON AND MANGANESE. 